Integrated circuits are very complex devices that include multiple layers. Each layer may include conductive material and isolating material, while other layers may include semi-conductive materials. These various materials are arranged in patterns, usually in accordance with the expected functionality of the integrated circuit. The patterns also reflect the manufacturing process of the integrated circuits.
Integrated circuits are manufactured by complex multi-staged manufacturing processes. During this multi-staged process, resistive material is (i) deposited on a substrate layer, (ii) exposed by a photolithographic process, and (iii) developed to produce a pattern that defines some areas to be later etched.
Various metrology, inspection and failure analysis techniques have evolved for inspecting integrated circuits both during the fabrication stages, between consecutive manufacturing stages, either in combination with the manufacturing process (also termed “in line” inspection techniques) or not (also termed “off line” inspection techniques). Various optical as well as charged particle beam inspection tools and review tools are known in the art, such as the VeraSEM™, Compluss™ and SEM Vision™ of Applied Materials Inc. of Santa Clara, Calif.
Manufacturing failures may affect the electrical characteristics of the integrated circuits. Some of these failures result from unwanted deviations from the required dimensions of the patterns. A “critical dimension” is usually the width of a patterned line, the distance between two patterned lines, the width of a contact and the like.
One of the goals of metrology is to determine whether the inspected objects include deviations from these critical dimensions. This inspection is usually done by charged particle beam imaging that provides the high resolution required to measure said deviations.
A typical measured structural element is a line that has two opposing sidewalls. The measurement of the bottom width of the line involves measuring the top width of the line as well as measuring its sidewalls.
Measurement of a structural element line's critical dimensions using only a top view (in which the electron beam that scans the line is perpendicular to the substrate) may result in faulty results, especially when one of the sidewalls has a negative sidewall angle such that an upper end of the sidewall obscures a lower end of that sidewall.
In order to address said inaccuracies, CD-SEM tools that enable electronic tilt of an electron beam were introduced. NanoSem 3D, of Applied Materials from Santa Clara, is a fully automated CD-SEM that has a column that allows electronic tilting as well as mechanical tilting of the scanning electron beam to scan the wafer surface with various tilt angles from several directions.
Critical dimension measurement may involve illuminating a test object by multiple tilted beams and processing the detected waveforms to define critical dimensions.
Multiple measurements have some disadvantages. First, they reduce the throughput of the inspection system, especially when the measurement involves changing the tilt of the scanning electron beam. Such a change may require a de-Gauess stage, as well as an electron beam stabilization stage. A further disadvantage of multiple measurements results from degradation (for example shrinkage and carbonization) of the measured structural element, as well as unwanted charging of the measured structural element.
Due to various reasons, such as process variations, measurement inaccuracies and the like, the height and accordingly the measured height of structural elements varies across measured objects. In order to determine the structural height of a measured structural element, there is a need to perform at least two measurements of said structural element, at two different tilt angles. The height of the structural element may be estimated, usually in response to multiple height measurements of structural element across the measured object. The estimation may be associated with height measurement errors, as well as estimation errors, that can affect the critical dimension measurements.